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Ch.19 - Chemical Thermodynamics
Brown - Chemistry: The Central Science 14th Edition
Brown14th EditionChemistry: The Central ScienceISBN: 9780134414232Not the one you use?Change textbook
All textbooksBrown 14th EditionCh.19 - Chemical ThermodynamicsProblem 97a
Chapter 19, Problem 97a

(a) For each of the following reactions, predict the sign of ΔH° and ΔS° without doing any calculations. (i) 2 Mg(s) + O2 (g) ⇌ 2 MgO(s) (ii) 2 KI(s) ⇌ 2 K(g) + I2(g) (iii) Na2(g) ⇌ 2 Na(g) (iv) 2 V2O5(s) ⇌ 4 V(s) + 5 O2(g)

Verified step by step guidance
1
Examine the reaction (i) 2 Mg(s) + O_2(g) ⇌ 2 MgO(s): This is a combustion reaction where a solid and a gas form a solid. Typically, such reactions are exothermic, so ΔH° is likely negative. The reaction results in a decrease in the number of gas molecules, suggesting a decrease in disorder, so ΔS° is likely negative.
Consider the reaction (ii) 2 KI(s) ⇌ 2 K(g) + I_2(g): This reaction involves the sublimation of a solid to form gases. Sublimation is endothermic, so ΔH° is likely positive. The increase in the number of gas molecules indicates an increase in disorder, so ΔS° is likely positive.
Analyze the reaction (iii) Na_2(g) ⇌ 2 Na(g): This reaction involves the dissociation of a diatomic gas into two monatomic gases. The process of breaking bonds is endothermic, so ΔH° is likely positive. The increase in the number of gas particles suggests an increase in disorder, so ΔS° is likely positive.
Evaluate the reaction (iv) 2 V_2O_5(s) ⇌ 4 V(s) + 5 O_2(g): This decomposition reaction involves the formation of a gas from a solid, which is typically endothermic, so ΔH° is likely positive. The increase in the number of gas molecules indicates an increase in disorder, so ΔS° is likely positive.
Summarize the predictions: For reaction (i), ΔH° is negative and ΔS° is negative. For reaction (ii), ΔH° is positive and ΔS° is positive. For reaction (iii), ΔH° is positive and ΔS° is positive. For reaction (iv), ΔH° is positive and ΔS° is positive.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Enthalpy (ΔH)

Enthalpy (ΔH) is a measure of the total heat content of a system at constant pressure. In chemical reactions, a negative ΔH indicates an exothermic reaction, where heat is released, while a positive ΔH indicates an endothermic reaction, where heat is absorbed. Understanding the nature of the reactants and products helps predict whether the reaction will release or absorb heat.

Entropy (ΔS)

Entropy (ΔS) is a measure of the disorder or randomness in a system. A positive ΔS indicates an increase in disorder, often occurring when gases are produced from solids or liquids, while a negative ΔS suggests a decrease in disorder. Evaluating the states of reactants and products in a reaction allows for predictions about changes in entropy.
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Entropy in Thermodynamics

Phase Changes and Reaction Direction

The direction of a chemical reaction and the associated changes in enthalpy and entropy can be influenced by the phases of the reactants and products. Reactions that produce gases from solids or liquids typically have positive ΔS, while reactions that form solids from gases may have negative ΔS. Recognizing these phase changes is crucial for predicting the signs of ΔH and ΔS in the given reactions.
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Related Practice
Textbook Question

(c) In each case, indicate whether K should increase or decrease with increasing temperature. (i) 2 Mg(s) + O2 (g) ⇌ 2 MgO(s) (ii) 2 KI(s) ⇌ 2 K(g) + I2(g) (iii) Na2(g) ⇌ 2 Na(g) (iv) 2 V2O5(s) ⇌ 4 V(s) + 5 O2(g)

Textbook Question

Using the data in Appendix C and given the pressures listed, calculate Kp and ΔG for each of the following reactions:

(a) N2(g) + 3 H2(g) → 2 NH3(g) PN2 = 2.6 atm, PH2 = 5.9 atm, PNH3 = 1.2 atm

(b) 2 N2H4(g) + 2 NO2(g) → 3 N2(g) + 4 H2O(g) PN2H4 = PNO2 = 5.0 × 10-2 atm, PN2 = 0.5 atm, PH2O = 0.3 atm

(c) N2H4(g) → N2(g) + 2 H2(g) PN2H4 = 0.5 atm, PN2 = 1.5 atm, PH2 = 2.5 atm

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Textbook Question

Consider the following three reactions: (i) Ti(s) + 2 Cl2(g) → TiCl4(1g) (ii) C2H6(g) + 7 Cl2(g) → 2 CCl4(g) + 6 HCl(g) (iii) BaO(s) + CO2(g) → BaCO3(s) (c) For each of the reactions, predict the manner in which the change in free energy varies with an increase in temperature.

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Textbook Question

Consider the following three reactions: (i) Ti(s) + 2 Cl2(g) → TiCl4(1g) (ii) C2H6(g) + 7 Cl2(g) → 2 CCl4(g) + 6 HCl(g) (iii) BaO(s) + CO2(g) → BaCO3(s) (b) Which of these reactions are spontaneous under standard conditions at 25 °C?

Textbook Question

The oxidation of glucose (C6H12O6) in body tissue produces CO2 and H2O. In contrast, anaerobic decomposition, which occurs during fermentation, produces ethanol (C2H5OH) and CO2.

(a) Using data given in Appendix C, compare the equilibrium constants for the following reactions:

C6H12O6(s) + 6 O2(g) ⇌ 6 CO2(g) + 6 H2O(l)

C6H12O6(s) ⇌ 2 C2H5OH(l) + 2 CO2(g)

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Textbook Question

(b) Based on your general chemical knowledge, predict which of these reactions will have K>1. (i) 2 Mg(s) + O2 (g) ⇌ 2 MgO(s) (ii) 2 KI(s) ⇌ 2 K(g) + I2(g) (iii) Na2(g) ⇌ 2 Na(g) (iv) 2 V2O5(s) ⇌ 4 V(s) + 5 O2(g)

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